I struggled a lot around this problem

code like this

include

void pippo () {
std::string s;
}

was generating error
undefined reference to `std::basic_string<char, std::char_traits, std::allocator >::~basic_string()

in order to fix it i had to force the compiler to generate the string class
i changed the string file
hardware\esp8266com\esp8266\tools\xtensa-lx106-elf\xtensa-lx106-elf\include\c++\4.8.2\string

into

//
`#ifndef _GLIBCXX_STRING

define _GLIBCXX_STRING 1

pragma GCC system_header

include <bits/c++config.h>

include <bits/stringfwd.h>

include <bits/char_traits.h> // NB: In turn includes stl_algobase.h

include <bits/allocator.h>

include <bits/cpp_type_traits.h>

include <bits/localefwd.h> // For operators >>, <<, and getline.

include <bits/ostream_insert.h>

include <bits/stl_iterator_base_types.h>

include <bits/stl_iterator_base_funcs.h>

include <bits/stl_iterator.h>

include <bits/stl_function.h> // For less

include <ext/numeric_traits.h>

include <bits/stl_algobase.h>

include <bits/range_access.h>

include <bits/basic_string.h>

include <bits/basic_string.tcc>

// in order to fix this
// cancme2.ino:(.text._Z5pippov+0x4): undefined reference to `std::basic_string<char, std::char_traits, std::allocator >::~basic_string()
// i had to force the compiler to generate the concrete class
template class std::basic_string < char > ;

endif /* _GLIBCXX_STRING */

`

and now works.

i wanted to commit the change but it seems that folder is not under git. how i can i contribute with this fix?
thks